Polymeric organic materials are widely used in all types of energetic formulations, primarily as either fuels or combustible binders. During the formulation of plastic bonded explosives, the hazard characteristics of all but the most insensitive of high explosives can be greatly improved by the addition of a suitable binder. However, whilst the addition of such a binder desensitises the explosive, if the binder is inert and has a lower density than the filler, it inevitably detracts from the performance. The tendency when formulating explosives is therefore to maximise solids loading in order to enhance performance. In contrast, larger quantities of binder are most beneficial in optimising safety. One way of improving these conflicting requirements is to use an energetic binder.
Energetic binders can still be effective in desensitising the explosive but are also able to contribute to the overall energy of the system. The consequence of this is that they can be used in somewhat larger proportions than an inert binder, whilst retaining, or even increasing, the overall energy of the system. Given that energetic polymers may be intrinsically less sensitive, enhanced quantities of these materials may benefit charge safety by two separate mechanisms: (1) through the attainment of reduced solids loading and (2) because of the intrinsic insensitively of the material being added. Thus, as the binder loading is increased, a non-detonable energetic binder is effectively replacing a proportion of the detonable crystalline filler. The term ‘energetic polymer’ is normally used to describe macro molecules which contain energetic functionalities such as nitrato, nitro or azido groups.
The difficulty with energetic binders is to obtain materials which combine high energy-density with peak physical properties. Existing examples of energetic binders comprise glycidyl azide polymer (GAP), poly (3-methyl-3-nitratomethyl oxetane) (polyNIMMO) and polyglycidyl nitrate (polyGLYN) share modest energy densities and relatively high glass transition temperatures (Tgs) which means that for service use they must be plasticised. It is a known problem that plasticisers tend to migrate out of explosives which can lead to the contamination of other components and compromise the low temperature performance of the explosive.
WO2006032882 describes an energetic binder which has both high energy-density and low glass transition temperature which could be used without plasticisation. This patent application clearly showed that polyphosphazenes could offer significant advantages over the energetic binder of the prior art in the formulation of reduced hazard compositions. However, the polyphosphazenes described in this application are not curable and could only be used, in pressed applications. This meant that plastic bonded explosives containing around 21% or more of the polyphosphazene binder would be likely to slump. Therefore, there is a need to cure the energetic binders in order to overcome the problem of a plastic bonded explosive slumping.
One existing class of energetic polyphosphazenes which contain azido substituted side chains has previously been cured (K. Bala, P. Golding, T. G. Hibbert, P. Jenkins, M. K. Till and M. Willcox, “Non-isocyanate curable, energetic (azido) polyphosphazenes” 41st International Conference of Fraunhofer ICT., Karlsruhe, Germany, 29 Jun. to 2 Jul. 2010) by reacting its pendant azido groups with a bismaleimide curing agent. Because these azide containing functionalities are the source of energy in this class of polymer, the number of such groups incorporated into the polymer chain is usually high (eg 50-100%) in order to maximise its energetic properties. However, the presence of high percentages of azide groups in such systems leads to disadvantages when using this type of cure procedure. Thus, when a difunctional curing agent (e.g. a bismaleimide) is added to an azido polyphosphazene of this type, it can react essentially in one of two ways: i) with two separate azide groups on different polymeric molecules (intermolecular reaction), thereby effecting cross-linking (i.e. the desired result) or ii) with two separate azide groups on the same polymer chain (intramolecular reaction). The latter reaction consumes reagent, but is unproductive as it links together different sections of a single polymer chain, without achieving a cure. Thus, reaction ii) consumes reagent without benefit, thereby requiring the addition of significantly more curative than is theoretically required to effect cross-linking of the polymer system. This has two disadvantages: 1) the excess curative is permanently incorporated into the polymer chain without benefit, but because it is inert it significantly decreases the overall energy (per unit mass) available from the system; 2) whenever the cure reagent reacts with azide groups, either in the desired mode, to produce cross-links or in the unproductive ‘intramolecular’ mode, it chemically destroys azide groups at both its reaction sites, as part of the cure process. Because there is a high percentage of azide groups present in the polymer (to maximise energy), the extent of intramolecular reaction becomes significant and hence the energy loss, due largely to removal of azide groups by these intramolecular reactions, also becomes significant.
A further disadvantage of this existing curing procedure is that it can only be effected with energetic polymers which contain azido functionalities. Azide based polymers, although energetic, are non-oxidising. When preparing energetic formulations (e.g. propellants, explosives and pyrotechnics) it is often difficult to achieve a good oxygen balance, particularly when using inert or non-oxidising energetic binders.
It is an object of the invention to provide curable polyphosphazenes which overcome or mitigate at least one of the above problems and/or another problem associated with the prior art.